The average gestation length for mares is 340 days although this can vary widely and can be as long as 360 days or more. Factors that may affect the length of gestation include sex of the foal, season, maternal nutrition, ingested toxins and environmental stresses.
Nearing the last month of gestation, the mare should start to be checked for physical changes that indicate nearness of delivery. These changes include development of vulvar laxity, relaxation of the pelvic ligaments, udder enlargement, and a change in the amount and character of mammary secretion.
Mammary gland growth becomes apparent most dramatically within the final 2 weeks of gestation. The accumulation of waxy secretion on teat ends from early colostrum formation usually occurs 1-4 days prior to foaling, although many of these parameters vary widely between mares as well as within individual mares depending on how many foals she has had previously. Some multiparous mares may even leak milk within the last couple of weeks of gestation, leading to a depletion of colostrum. Often, secretions immediately prior to foaling have a thick, sticky consistency and can be more yellow than white in color.